01 March 2023
Salvias have become valuable contributors to the urban landscape in recent decades, notably the many cultivars of Salvia nemorosa (S. x sylvestris) valued for their rich blue tones, long flowering and pest resistance. Reliably hardy salvias used in urban plantings tend to be herbaceous and of European or Asian origin, preferring free draining, relatively rich soils.
Currently under-explored are some of the woody American/Mexican species whose performance under significant heat and water stress offers significant landscape potential in the face of increasingly extreme urban summers. Healthy, resilient, and potentially long-lived when grown in poor mineral soils, they are ideally suited to Green City contexts, particularly when using engineered substrates.
Aromatic like their Eurasian cousins, instead of a “sagey” herbal scent, the leaves of these New World species tend to emit a sweeter, floral or fruity fragrance when bruised or brushed against, adding sensory enrichment to the plant-people interactions that promote a sense of well-being in built-up environments.
Pollinated in the wild by bees and hummingbirds, their nectar-rich flowers are potentially valuable in addressing the urban “nectar drought” warned of by city bee-keepers and ecologists – very relevant given the increasing prominence of biodiversity requirements in urban design briefs. (Italian researchers have observed native bee species exploiting floral resources from exotic salvias.)
The catch is that many of the New World species (e.g. Salvia greggii) do not reliably survive cold winters unprotected. However, recent data from Germany and the UK suggests that several are hardier than generally realised: the low-growing, strongly aromatic Salvia lycioides (canyon sage), with deep violet-purple flowers, has proven hardy to -18°C; the tall, orange-red Salvia darcyi to -15°C; and the blue-flowered ground-cover Salvia chamaedryoides to -12°C. These New World salvias are worth closer attention, particularly if an advantage can be taken of local microclimates and the protection afforded by buildings.
By Martin Deasy, a tutor on the RHS Mhort. He also runs his own landscape design business.